A098450 Largest n-digit semiprime.

9, 95, 998, 9998, 99998, 999997, 9999998, 99999997, 999999991, 9999999997, 99999999997, 999999999997, 9999999999989, 99999999999997, 999999999999998, 9999999999999994, 99999999999999989, 999999999999999993, 9999999999999999991, 99999999999999999983

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..100

Formula

a(n) = 10^n - A119320(n). - Amiram Eldar, Sep 18 2021

Mathematica

NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[ PrimeOmega[sp] != 2, If[sgn < 0, sp--, sp++]]; If[sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]]; f[n_] := NextSemiPrime[10^n, -1]; Array[f, 18] (* Robert G. Wilson v, Dec 18 2012 *)

Prog

(PARI) apply( A098450(n)={n=10^n; until(bigomega(n-=1)==2, ); n}, [1..20]) \\ M. F. Hasler, Jan 01 2021
(Python)
from sympy import factorint
def semiprime(n): f = factorint(n); return sum(f[p] for p in f) == 2
def a(n):
  an = 10**n - 1
  while not semiprime(an): an -= 1
  return an
print([a(n) for n in range(1, 21)]) # Michael S. Branicky, Apr 10 2021

Crossrefs

Cf. A098449 (smallest n-digit semiprime), A003618 (largest n-digit prime), A001358 (semiprimes), A119320.

Sequence in context: A000562 A193216 A213019 * A337269 A065456 A052389

Adjacent sequences: A098447 A098448 A098449 * A098451 A098452 A098453

Keyword

base,nonn

Author

Rick L. Shepherd, Sep 07 2004

Status

approved